Racial microaggressions are the brief and everyday slights, insults, indignities and denigrating messages sent to people of color by well-intentioned White people who are unaware of the hidden messages being communicated. These messages may be sent verbally ("You speak good English."), nonverbally (clutching one's purse more tightly) or environmentally (symbols like the confederate flag or using American Indian mascots). Such communications are usually outside the level of conscious awareness of perpetrators.
If you are not familiar with the concept of microagressions, the article where this quotation comes from is a helpful primer.